This invention relates to an electrical interface circuit and particularly, though not exclusively, to an interface circuit providing a high impedance input for a sensor being read and providing compensation for ground shift errors in the sensor output.
A zirconium dioxide sensor (sometimes referred to as a lambda, or oxygen, sensor), commonly used in automobile engine/fuel-control systems, produces an output voltage (in the range of 0 to 1 volt) representative of combusted air/fuel mixture. Such a sensor has an extremely variable effective output resistance (from tens of MegOhms to hundreds of Ohms) as a function of sensor element temperature. In one common construction of a zirconium dioxide sensor, the sensor element is designed to be electrically connected to the mechanical sensor mounting body, making the sensor ground at the same potential as the engine block ground. However, as is well known, due to different current ground return paths in a vehicle, an electronic module's ground potential (commonly referred to as "circuit" ground) may differ from the actual engine block ground potential by as great as .+-.1 volt. Thus, an interface circuit for reading a "grounded body" zirconium dioxide sensor output voltage must discriminate between the sensor output voltage and any ground shift, and must be unaffected by the sensor's wide range of output impedance. Thus, such an interface circuit must be both high impedance and have differential characteristics.
Unfortunately, a typical simple single operational amplifier differential circuit does not have a suitably high enough input impedance for the use with a zirconium dioxide sensor, and a typical simple single operational amplifier high impedance input circuit can not easily be made to adequately perform differentially, particularly when the sensor output voltage referenced to the engine ground is less than the circuit ground. Thus, these typical operational amplifier circuits are not suitable for use with a "grounded body" zirconium dioxide sensor.
Also, in some applications it is desirable for the interface circuit to have some input to output gain capability. However, increasing the gain can limit the interface circuit's ground shift rejection capability, and compromises must be made in the circuit design to limit this undesired interaction.